THIS INVENTION relates to the treatment of sugar juice. It relates in particular to a process for treating impure cane-derived sugar juice, typically raw juice which has been subjected to conventional preclarification by heating, liming and settling.
According to the invention, there is provided a process for treating impure cane-derived sugar juice, which process comprises
subjecting, in a clarification stage, impure cane-derived sugar juice to microfiltration/ultrafiltration to decrease the levels of suspended solids, organic non-sugar impurities and/or colour therein;
sequentially passing the resultant clarified sugar juice through at least one ion exchange stage by bringing the clarified sugar juice into contact with a strong acid cation ion exchange resin in the hydrogen form, and thereafter into contact with an anion ion exchange resin in the hydroxide form;
withdrawing a purified sugar solution from the ion exchange stage;
concentrating the purified sugar solution, to produce a syrup;
subjecting the syrup to primary crystallization in at least one primary crystallization stage, to produce refined white sugar and primary mother liquor or molasses;
subjecting the primary mother liquor to secondary crystallization in at least one secondary crystallization stage, to produce impure crystallized sugar and secondary mother liquor or white strap molasses.
The impure cane-derived sugar juice typically is that obtained by preparing sugarcane stalks, eg disintegrating or breaking up the stalks; removing sugar juice from the prepared stalks by diffusion and/or milling, using imbibition water, thereby to obtain mixed juice; heating and liming the mixed juice; and subjecting it to primary clarification, to obtain clear juice, ie to obtain the impure cane-derived sugar juice which constitutes the feedstock to the process of the invention. Instead, however, the clear juice or impure cane-derived sugar juice which is used as feedstock can be that obtained by any other suitable preparation process.
The impure cane-derived juice is typically at an elevated temperature, eg a temperature above 90xc2x0 C. Thus, the microfiltration/ultrafiltration will also be effected at elevated temperature; however, since ion exchange normally takes place at a lower temperature, eg at a temperature below 60xc2x0 C., such as at about 10xc2x0 C., the juice will normally be cooled before ion exchange.
The impure sugar juice as obtained from sugar cane stalks as hereinbefore described, has a low sugar or sucrose concentration, typically less than 15% (m/m), for example in the order of 10% to 15% (m/m). This low concentration impure sugar juice is suitable as a feedstock for the process of the present invention; however, it is believed that it will be advantageous to use a higher concentration impure sugar juice as feedstock, eg to reduce the cost of the capital equipment required to treat the same amount of sugar or sucrose. Thus, the process may include concentrating, eg by means of evaporation, the impure sugar juice before it enters the clarification stage. It may be concentrated to a sugar or sucrose concentration of at least 20% (m/m), preferably from 20% to 40% (m/m), typically about 25% (m/m).
The impure cane-derived sugar juice will thus normally, during preparation thereof, have been subjected to initial or primary clarification; the treatment in the clarification stage of the process of the invention thus constitutes secondary clarification of the sugar juice. In the secondary clarification stage, sufficient suspended solids, organic non-sugar impurities and colour are removed to render the sugar amenable to subsequent treatment in the ion exchange stage. During the secondary clarification, the sugar juice may be passed through a membrane in the size range 15000 Dalton to 300000 Dalton or 200 Angstrom to 0,2 micron. The Applicant has found that microfiltration/ultrafiltration prior to ion exchange is important in order to inhibit rapid fouling of the ion exchange resins, and to ensure that the refined white sugar product meets the required turbidity specifications.
In the ion exchange stage, de-ashing or demineralization and further colour removal takes place. The contacting of the clarified sugar juice with resins is effected in such a manner that inversion, ie breakdown of sucrose to glucose and fructose is kept as low as possible, and resin use is optimized.
In certain circumstances, strong acid cation resins can catalyze the inversion reaction of sucrose. To inhibit sucrose inversion in such cases, the ion exchange, or a portion of the ion exchange, can be effected at sugar juice temperatures below 30xc2x0 C. The process may thus include, when necessary, reducing the impure sugar juice temperature to below 30xc2x0 C., ahead of or during its passage through the ion exchange stage. For example, the sugar juice temperature can be reduced to about 10xc2x0 C., eg by using a refrigeration plant, to ensure minimal sucrose inversion.
The ion exchange stage may be provided by a simulated moving bed arrangement or system, eg by a continuous fluid-solid contacting apparatus such as that described in U.S. Pat. No. 5,676,826; by a separation train system such as that described in U.S. Pat. No. 5,122,275; or the like.
The process may include subjecting the clarified sugar juice to a first pass through the ion exchange stage, to obtain a partially purified sugar solution, and thereafter subjecting the partially purified sugar solution to at least one further pass through the ion exchange stage, to obtain the purified sugar solution.
The process includes regenerating the resins from time to time, as required. Thus, the strong acid cation resin may be regenerated by contacting it with a strong acid, such as hydrochloric acid or nitric acid, with an acid stream rich in potassium salts thereby being obtained. This component is suitable for use as a fertilizer feedstock. The anion resin may be regenerated by contacting it with a strong or weak base such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, or a combination of sodium or potassium hydroxide and ammonium hydroxide, with an alkaline stream which is rich in nitrogen being obtained. This component is also suitable for use as a fertilizer feedstock.
As indicated hereinbefore, de-ashing or demineralization (cations and anions) and colour removal are effected simultaneously in the ion exchange stage. However, the Applicant has found that it is not always the most efficient route to remove all colour during passage of the sugar juice through the ion exchange stage. Some colour may thus, if desired, be removed in the ion exchange stage, with the remaining colour then being removed by further treatment of the sugar juice.
Thus, in one embodiment of the invention, the process may include subjecting the purified sugar solution from the ion exchange stage, or the partially purified sugar solution of the ion exchange stage, to further decolourizing in a decolourizing stage.
The decolourizing stage may comprise an anion resin, in particular an anion resin in hydroxide or chloride form; an absorption resin; activated carbon; or another absorption medium.
When the decolourizing stage includes an anion resin in the chloride form, the partially purified sugar solution, after the first pass thereof through the ion exchange stage, may be brought into contact with the anion resin in the chloride form in the further ion exchange stage, and thereafter subjected to a second pass through the ion exchange stage.
When the decolourizing stage includes an anion resin in the hydroxide form, an absorption resin, activated carbon, or another absorption medium, the purified sugar solution from the ion exchange stage may be brought into contact with the anion resin, the absorption resin, the activated carbon or the other absorption medium.
The concentration of the purified sugar solution into the syrup may be effected by means of evaporation. The resultant syrup may have a sucrose or sugar concentration of about 65% (m/m).
The primary crystallization may be effected in a plurality of sequential primary stages or boilings. The secondary crystallization may also be effected in a plurality of sequential primary stages or boilings. The purge or mother liquor from the primary crystallization is thus exhausted further by the secondary crystallization to recover the impure sugar crystals. The impure crystallized sugar from all the secondary crystallization stages or boilings may be remelted or redissolved, and recycled to the syrup ahead of the primary crystallization stages. This recycle is typically less than 20% of the total feed to the primary crystallization stages. The purge or mother liquor from the secondary or exhaustion crystallization stages is thus defined as the white strap molasses.
In another embodiment of the invention, the process may include subjecting the syrup, prior to the primary crystallization, to decolourizing crystallization in a decolourizing crystallization stage, to produce high colour white sugar and tertiary molasses; remelting or redissolving the high colour white sugar to produce a remelted sugar solution which is then subjected to the primary crystallization in the primary crystallization stages; returning the primary mother liquor or molasses produced in the primary crystallization stages to the decolourizing crystallization stage; subjecting the tertiary molasses from the decolourizing crystallization stage to mill crystallization in a mill crystallization stage to produce the white strap molasses and impure crystallized low colour sugar; and returning the impure crystallized low colour sugar to the decolourizing crystallization stage, with the decolouring crystallization and the mill crystallization constituting the secondary crystallization.
The white strap molasses is a low ash material suitable for various uses, eg for fermentation, for the manufacture of high purity by-products, can be subjected to chromatographic separation for recovery of sucrose, or can be used as a liquid sugar source. Thus, the white strap molasses is a secondary high value product. The white strap molasses has, without further processing thereof, the following typical properties:
sucrose content of less than 40% on a dry solids basis;
sugar (sucrose, glucose and fructose) content of more than 75% on a dry solids basis, with the sucrose fraction depending on the ion-exchange stages;
ash (inorganic material) content of less then 2,0%;
organic non-sugars of less than 24%.
The invention extends also to the products obtained from the process of the invention, ie a potassium-rich acid stream or component, a nitrogen-rich alkaline stream or component, white strap molasses, and refined sugar, when produced by the process of the invention.
The Applicant has unexpectedly found that by subjecting impure cane-derived sugar juice to microfiltration/ultrafiltration and subsequent ion exchange in accordance with the invention, removal of substantially all the colour and turbidity which is present in the impure cane-derived sugar juice is achieved. A purified sugar solution suitable for the direct production of white or refined sugar without any pre-crystallization or raw sugar house treatment thereof being required, is thereby obtained.
By contacting the clarified sugar juice with a strong acid cation exchange resin in the hydrogen form followed by an anion exchange resin in the hydroxide form, substantially all inorganic ions are removed; however, it was also unexpectedly found that in excess of 60% of the organic non-sugars present in the sugar juice are also thereby removed. This thus means that in excess of 70% of the molasses non-sugar components are removed by the ion exchange, which leads to higher overall recovery of sucrose if sucrose inversion is minimized, as herein described.
To minimize inversion of sucrose to glucose and fructose, the ion exchange is, as hereinbefore described, preferably effected in a simulated moving bed and at a low temperature. The simulated moving bed allows the acid released to be neutralized as the juice passes through the ion exchange bed, and also reduces the residence time. It was thus surprisingly found that by subjecting the sugar juice to ion exchange in a simulated moving bed, having at least one pass, at about 50xc2x0 C., or at an even lower temperature, eg at about 10xc2x0 C., in certain cases, the inversion is reduced to less than 1%.
To obtain both low inversion and sufficient colour and non-sugar impurity removal is critical in order to achieve an economically viable process.